1. Field of the Invention
The present invention relates to a process for the preparation of praseodymium (Pr) metal, a praseodymium-neodymium (Pr-Nd) alloy, or an iron alloy thereof (Pr-Fe or Pr-Nd-Fe). More particularly, the present invention relates to a process for preparing the above-mentioned metal or alloy by the fused salt electrolytic method using praseodymium fluoride or praseodymium fluoride and neodymium fluoride as the starting material. Especially, the present invention relates to a process in which a high-purity metal or alloy suitable for a magnetic material of the Pr type or Pr-Nd type, which recently has attracted attention for high-performance magnets, can be manufactured at a low cost.
2. Description of the Related Art
As a relatively cheap high-performance permanent magnet, there has recently been proposed a magnet formed by substituting a part of the Sm type magnet composition by Pr or a Pr-Nd alloy, or a permanent alloy of the (Pr, Nd)-Fe-B type or (Pr, Nd)-Fe-Co-B type. As the method for preparing Pr, Pr-Nd, or an iron alloy thereof to be used for such permanent alloys, the following methods are known.
(1) A method comprising reducing a Pr compound or Pr-Nd compound with an active metal such as metallic calcium.
(2) A method comprising carrying out alloying reaction between Pr oxide or Pr-Nd oxide and iron used as the cathode by fused salt electrolysis to collect Pr, Pr-Nd, or an Fe alloy thereof (E. Morrice et al, Bur. Mine Rep. Invest., No. 7146, 1968).
(3) A method comprising carrying out an alloying reaction between Pr fluoride or Pr-Nd fluoride and iron used as the cathode by the fused salt electrolysis to collect a Pr-Fe or Pr-Nd-Fe alloy (Japanese Unexamined Patent Publication No. 61-253391).
These three methods are now compared with one another. The first method is disadvantageous from the economical viewpoint and in view of the low productivity because an expensive metal such as calcium is used as the reducing agent and the reaction is carried out batchwise.
In the second method, since the solubility of the oxide in the fused salt is low, if the oxide is fed in an amount exceeding the solubility, the oxide is incorporated in the lower portion of the fused salt, that is, in the deposited metal, to form a mixture of the metal with the fused salt and oxide, and the second method is defective in that recovery of a high-grade metal is difficult.
In the third method, the dissolution of the fluoride in LiF as the fused salt forms a eutectic mixture and the dissolution range is broad. The metal can be recovered without trouble as observed in the oxide electrolysis method. Accordingly, the third method is excellent in this point.
However, as the result of investigations made by the inventor, it has been confirmed that the method disclosed in Japanese Unexamined Patent Publication No. 61-253391 has problems described below and is not suitable for preparing a Pr-Fe or Pr-Nd-Fe alloy economically advantageously on an industrial scale.
According to the method disclosed in Japanese Unexamined Patent Publication No. 61-253391, Pr fluoride or a mixture of Pr fluoride and Nd fluoride is maintained at a concentration of 35 to 76% by weight in an electrolytic bath composed mainly of LiF and a Pr-Fe or Pr-Nd-Fe alloy is deposited while maintaining the temperature of the electrolytic bath at 770.degree. to 950.degree. C. The reason why the amount of PrF.sub.3 as the starting compound or the starting PrF.sub.3 -NdF.sub.3 mixture is limited to 35 to 76% by weight is as follows. The phase diagram of LiF-PrF.sub.3 shown in FIG. 5 is the eutectic phase diagram. The melting points of LiF, PrF.sub.3, and LiF-PrF.sub.3 at the eutectic point (the composition where the amount of PrF.sub.3 is about 67% by weight) are about 850.degree. C., higher than about 1300.degree. C. and about 730.degree. C., respectively. Accordingly, it is considered that the above-mentioned composition of the LiF-PrF.sub.3 system is selected so that the melting point of the bath is lower than about 840.degree. C. The phase diagram of the LiF-NdF.sub.3 is substantially the same as described above, and the same can be said with respect to the bath composition for the production of a Pr-Nd-Fe alloy (R. E. Thoma, Progress in Science and Technology of the Rare Earths, Vol. 12, page 110, Pergamon Press, New York, (1966). In the production of a metal by the fused salt electrolytic method, for the reasons set forth below, it is preferred that the electrolysis temperature be lower. Accordingly, it is considered that the composition of the fused salt be such that the operation is carried out on the fused salt having a low melting point. It is considered that the above-mentioned composition is selected from this viewpoint.
(1) When the operation is carried out at a low temperature, the damage of the electrolytic cell material is small, and also the evaporation loss of the electrolytic bath is small.
(2) An energy-saving effect is attained when the operation is carried out at a lower temperature.
3In the fused salt electrolysis, the once deposited metal is formed into metal mists again and these metal mists react with the bath components to form the starting material. This reaction is conspicuous as the temperature is higher. It is considered that the ratio of the actually recovered amount of the metal to the theoretical value, calculated based on this balance, that is, the current efficiency, is reduced as the temperature is higher.
However, as the result of investigations made by us, it was confirmed that in the fluoride electrolysis, unlike the known oxide or chloride electrolysis, the reaction is advanced through a complicated mechanism, and the above-mentioned composition is not economically desirable.
The following two points are important for producing a metal economically advantageously by the fused salt electrolysis.
(1) Critical Current Density
In the fused salt electrolysis, as the current per unit area of the electrode increases, a peculiar phenomenon called "the anode effect" arises. In order to perform the electrolysis stably, it is ordinarily necessary to carry out the operation at a current density lower than the critical current density. If the critical current density is low, in order to apply a certain quantity of an electric current, it is necessary to increase the electrode area, and hence, the size of an electrolytic cell in which electrodes are placed should be increased. This results in increase of the size of the furnace and increase of the equipment cost. A large quantity of expensive fused salt should be used, and the production becomes economically disadvantageous. Therefore, in order to produce a metal economically advantageously, it is preferable to carry out the operation under a condition of a high critical current density. According to our research, it was found that in the electrolysis for the production of rare earth metals, since this critical current density is not substantially influenced by the cathode current density but is mainly determined by the anode current density, it is preferable to carry out the operation under a condition where the anode current density is substantially high.
(2) Current Efficiency
As pointed out hereinbefore, the quantity of the metal produced by the electrolysis is calculated by multiplying the theoretical value calculated by Faraday's law by the current efficiency. If the current efficiency is low, the amount produced of the metal does not increase in proportion to the quantity of the electric current applied, and therefore, a higher current efficiency is desired.
Under this background, the inventor made research with a view to developing a process capable of producing a metal economically advantageously, and, as a result, the inventor found that the conditions disclosed in Japanese Unexamined Patent Publication No. 61-253391 mentioned above are not the absolutely optimum conditions and Pr-Fe and Pr-Nd-Fe alloys can be prepared very economically advantageously under different conditions. The inventor has now completed the present invention based on this finding.